Recently, the usage of non-volatile memory comprising a semiconductor device capable of rewriting data and retaining data stored therein even while the device is un-powered has become increasingly popular. Flash memory is one example of non-volatile memory. A typical flash memory is provided with a transistor which operates as a memory cell and includes a floating gate or an insulating film called a charge storage layer for accumulating electrons used to store data. Flash memory with a SONOS (Silicon Oxide Nitride Oxide Silicon) structure for accumulating the electrons in a trap layer of an ONO (Oxide Nitride Oxide) film has been introduced as a flash memory using an insulating film as the charge storage layer.
A technique for forming the ONO film on the side surface of a groove formed in the semiconductor substrate has been developed to miniaturize the memory cell. PCT International Application No. 2005-525695 discloses a method for manufacturing the flash memory having the aforementioned groove as described below. According to PCT International Application No. 2005-525695, the trench is formed in the semiconductor substrate while using the auxiliary layer formed of the nitride film as a mask. Thus, the ONO film is formed on the wall of the trench. The gate electrode (formed of polysilicon) is formed in the trench. The auxiliary layer is eliminated to form the source/drain region in the semiconductor substrate while using the gate electrode as the mask. The insulating film is then formed on the source/drain region and the word line is formed on the insulating layer. The source/drain region may be formed after forming the ONO film. The insulating layer may be used to electrically separate the source/drain region from the word line.
However, according to this technique, the trap layer constituting the ONO film is eroded while eliminating a dummy layer used as a mask to form the groove. The level of the erosion varies depending on the individual wafer or the manufacturing lot. Occasionally, the trap layer may become sufficiently eroded so as to interfere with the stable operation of the flash memory.